If the MacAir is the source, then you don’t connect to the Oppo. Connect the computer directly to the DAC by usb.
Note that the Oppo 205 can be used as the external DAC for the Mac, but if you go this route, do not then connect the Oppo to another external DAC
Note that the Oppo 205 can be used as the external DAC for the Mac, but if you go this route, do not then connect the Oppo to another external DAC